The invention relates to switching power apparatus, particularly for power supply systems providing an AC or DC output signal. The apparatus has high efficiency, high reliability and very small number of components. A power transformer is used if line isolation is necessary. The apparatus can be employed in a switching power supply (SPS), switching power amplifier, uninterruptible power supply (UPS), programmable converter, line conditioner, AC voltage converter, frequency converter, etc.
Power amplifiers are devices designed to amplify an input signal and provide an undistorted output signal which is independent of supply voltages, load fluctuations over frequency, operating temperature, etc. In particular, switching power amplifiers provide significantly higher efficiency by employing high speed switches. They are often used to provide signal having a fixed frequency, as in UPS system, frequency converter, etc.
Conventional SPSs with AC output, i.e. switching power amplifiers, perform multiple conversions. The line voltage is rectified. The rectified voltage is stored in one or two capacitors. In one type of the SPSs, the capacitor voltages are converted into low frequency signal which has the desired waveform. A transformation is performed which however is inefficient for low frequencies. The transformer has to be designed for high inductance and is relatively of large size and heavy weight. This does not correspond to modern electronic construction which is usually of high packing density with reasonable weight. In another type of the SPSs, the capacitor voltages are converted into high frequency AC signal which is transformed. Output signal of the transformer is rectified and voltages of opposite polarities are stored in two capacitors. These voltages are converted into the AC output signal which has the desired waveform. The SPSs employ the isolation transformers whether or not line isolation is required.
The disadvantages of conventional switching power amplifiers are many. The input signal, e.g. audio, is usually converted into a specific pulse width modulated signal (PWM) which is a combination of analog and digital signals. The pulse width can have any value within a switching period and is influenced by all kinds of errors. Each of the complementary output transistors must switch at least once within the high frequency period. A poor tracking of the transistor switching times results in asymmetric errors in pulse width. Power supply ripples and pulse amplitude errors depend on output power which increases distortion level, particularly near crossover point. Pulses are lost near extremes of modulation. A dead time of the transistors is inevitable in order to prevent overlapping of the conduction phases thereof due to drive asymmetry, poor transistor recovery characteristics or inadequate deadband of a control circuitry.
Furthermore, the output filter in resonant configuration is designed for one specific frequency. Its performance is poor due to regulation concepts, such as the PWM, rather than practical limitations of the components. A very high switching frequency is necessary, mostly tens of times higher than a maximum frequency of the amplified signal. This results in a further reduced efficiency and a relatively high distortion level due to the limited switching performance of the power switches. Another unsolved problem is a desirable load having a relatively stable impedance over frequency. The equivalent switching frequency of the individual transistors is actually significantly higher as the switching times thereof are a fraction of the switching period. An SPS for stabilizing supply voltages is common. This further increases the complexity and in most cases generates a huge amount of EMI/RFI, especially into the line. Numerous interference suppressors and protection circuits ar inevitable.
Harmonic distortion level is defined as 100% times the ratio of the RMS sum of the harmonics to the fundamental. Therefore, the harmonic distortion level is inversely proportional to the fundamental. Output currents of the output switches of the switching power amplifier usually increase with increasing output voltage. Therefore, it is desirable to minimize the switching of the output switches with increasing output voltage level as to maximize the amplifier efficiency but with no significant deterioration of the harmonic distortion level.